I don't think that is correct. If the primary 02 sensor is working then the car should run the same whether or not the pre-cat or second 02 sensor is bad. I'll have to take a look @ my wife's car to verify but I don't know why Mazda would force your car into open loop if the primary 02 sensor is still good.
I was kinda speaking to whatusername specifically (he's running without a pre-cat,...I guess I should have PM'ed him) but I think (this is just my observation) the second O2 is tied in with the closed loop parameters. When I watch my first and second O2 readings with my Ultra-Gauge it really looks like the second O2 sensor has something to do with the fuel trim and is being monitored by the ECU for more than just whether or not the pre-cat is working. Both Installshield and the service manual say the second O2 sensor is only present to monitor the pre-cat but I think otherwise. I think I'll maybe disconnect my second O2 to see what kind of numbers I get but that will set off my CEL which should set the ECU to run open loop (my Ultra-Gauge doesn't ever show closed loop, so I'll just have to watch my O2 and fuel trim numbers). If the second O2 sensor has nothing to do with fuel trim and open/closed loop, then I don't see why the exhaust should stink and get carboned up as well as a drop in gas mileage when people delete their pre-cat and install the non-foulers.
In an effort to bring this back to the OP,... I think the O2 sensors can very well have an impact on MPG and overall drive-ability, although there is 11 different CEL codes that cover the O2 sensors and pre-cat, so you would think that you would get some kind of code if they were bad.
I would like to know if O2 sensors in general "wear out" over time and can go out of spec. but still function enough to not set off a CEL,... maybe kinda like spark plugs becoming weak with age. At around $22 each or so, it doesn't seem like too much money to spend to make sure.
I'm consistently getting between 28 and 32 MPG (US gallons) and only once dropped down to 25 MPG when my air filter was dirty.
Here's what wikipedia has to say about O2 sensor failure.
Sensor failures:
Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 miles (50,000 to 80,000 km). Heated sensor lifetime is typically 100,000 miles (160,000 km). Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burned off during operation and failure occurs due to catalyst depletion. The probe then tends to report lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the fuel economy worsens.
Leaded gasoline contaminates the oxygen sensors and catalytic converters. Most oxygen sensors are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 15,000 miles depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty.
Another common cause of premature failure of lambda probes is contamination of fuel with silicones (used in some sealings and greases) or silicates (used as corrosion inhibitors in some antifreezes). In this case, the deposits on the sensor are colored between shiny white and grainy light gray.
Leaks of oil into the engine may cover the probe tip with an oily black deposit, with associated loss of response.
An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel rationing system.
Applying an external voltage to the zirconia sensors, e.g. by checking them with some types of ohmmeter, may damage them.
Some sensors have an air inlet to the sensor in the lead, so contamination from the lead caused by water or oil leaks can be sucked into the sensor and cause failure.[4]
Symptoms of a failing oxygen sensor includes:
Sensor Light on dash indicates problem
Increased tailpipe emissions
Increased fuel consumption
Hesitation on acceleration
Stalling
Rough idling